'I worked a lot harder today for my 71 than I did for my 66 on Thursday or the 72 on Friday,' said Thatcher. 'I felt like I had to make a lot more saving putts today.'
Thatcher began the day with a one-shot lead over Jason Caron at the difficult Bayonet Course and slowly distanced himself from his competitors thanks to steady play. He picked up his first birdie of the day at the fourth and added another at the seventh to reach 8-under.
The 25-year-old ran into trouble with a bogey at the ninth, but countered with a 40-footer for birdie at the 11th.
'That's the longest putt I've made all week,' said Thatcher. 'I hadn't come close to hitting a long one all week, but you're not supposed to make those bombs. If you're 40 feet from the hole you haven't done your job. You should try to get two putts and move on.'
Despite a bogey at the par-3 14th, Thatcher had a chance to increase his lead to six strokes, but the Auburn University grad three-putted the closing hole for a par.
'I kind of let one get away at the last hole,' said Thatcher. 'But then again, I stole one or two earlier in the day. I just missed an opportunity to extend the lead.'
Thatcher is seeking his first career victory on the Buy.Com Tour. The rookie posted his only top-10 finish of the season at the Louisiana Open, the third event of the year, and struggled from that point on, including six straight missed cuts prior to this week.
Aaron Baddeley shot a 70 to finish tied for second alongside Anthony Painter at 2-under-par 214. Baddeley was within four of the lead, but bogeyed the par-5 18th.
'I would have liked to make birdie there to put some pressure on for tomorrow,' said Baddeley. 'Anything can happen though.'
Barry Cheesman and Charles Warren were one shot further back at 1-under-par 215.
Dave Schreyer, Vic Wilk and Omar Uresti share sixth place at even-par 216.
Jason Buha shot a 3-under 69 to join Joel Kribel in a tie for ninth at 1-over-par 217.
Full-field scores from Monterey Peninsula Classic